ANALYZING DEFECTS IN WELDMENTS USING METALLOGRAPHY EXAMINATION

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1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 1, Issue 2008, Jan Dec 2008, pp Available online at Journal Impact Factor (2008): (Calculated by GISI) ISSN Print: and ISSN Online: IAEME Publication ANALYZING DEFECTS IN WELDMENTS USING METALLOGRAPHY EXAMINATION Dr. S. Ravichandran CEO & Chief Scientist, Trimentus Technologies, T. Nagar, Chennai, India ABSTRACT Austenitic stainless steels are some of the most widely used materials in modern technology especially in industries like nuclear, chemical, petrochemical, aerospace and defence. This is so because they have an excellent combination of mechanical properties and resistance to corrosion. For critical applications, these materials have to be free from defects to assure good performance. The austenitic stainless steels are also welded with carbon steel materials called austenitic-ferritic dissimilar metal welds (DMW), widely used in superheaters and reheaters of power plants and in nuclear services. Key words: Weldment and Defects INTRODUCTION Many grades of austenitic stainless steels are extensively used in nuclear, space, chemical, and petrochemical industries. Austenitic stainless steel weldments used in these demanding service conditions must be subjected to stringent testing/nondestructive evaluation for assuring desired quantity. Early detection of defects in these weldments, before they grow are detected by conventional ultrasonic and radiography testing methods. Radiographic testing of these weldments has limitations posed by weld geometry and accessibility of both sides of the weld for inspection. The Austenitic stainless steel weldments are known for the strong attenuation of the ultrasound. The attenuation of ultrasound energy is due to the dendritic textured/columnar microstructure present in the weld and the consequent scattering in the grain boundaries. This results in a decrease in the signal to noise (S/N) ratio and as the thickness of the weldment increases, the reduction in the signal to noise ratio is appreciable editor@iaeme.com

2 Analyzing Defects In Weldments Using Metallography Examination EXPERIMENTAL PROCEDURES Pattern Recognition Approach AISI type 304 austenitic stainless steel weld plates were used for this study. Typical chemical composition of austenitic stainless steel is shown in Table 1. The plates were ascertained to be defect free by employing ultrasonic testing before welding. TABLE 1 Chemical Composition of AISI Type 304 Austenitic Stainless Steel Element By % Weight Carbon 0.08 Chromium 19.0 Nickel 10.2 Nitrogen 0.03 Manganese 2.0 Silicon 1.0 Phosphorous Sulphur 0.03 Iron Balance CARBON STEEL Carbon steel plates were used for comparing the ability of non-destructive evaluation of carbon steel weldment with austenitic stainless steel weldments. The plates were ascertained to be defect free by employing ultrasonic testing before welding. Chemical composition is given in Table 2. Table 2 Chemical Composition of Carbon Steel Type A 106 GRADE B Element By % Weight Carbon 0.28 Manganese 0.50 Sulphur 0.05 Phosphorous 0.05 Silicon 0.08 Iron Balance 15 editor@iaeme.com

3 Dr.S. Ravichandran 2.1 WELD PREPARATION AISI Type 304 Austenitic Stainless Steel Weldment Six austenitic stainless steel plates having thickness 1 mm, 17.5 mm, 25.0 mm, 36.0 mm, 40.0 mm and 50 mm were fabricated for this study. The welding process used for the fabrication of these plates is shielded metal arc welding (SMAW). Single - V (75 ) butt welding was carried out in all the plates as per ASME Boiler and pressure vessel code (96). After the welding process, the plates were machined. The plates (25.0 mm and 50.0 mm thick weldments in which artificial defects were planned to be incorporated) where chosen to be free from natural defects. The other plates have natural defects produced by varying welding parameters. The defects include porosity, lack of penetration and lack of fusion. Weld parameters chosen for incorporating these natural defects were employment of higher current and by improper cleaning of these plates while welding them. The welding parameters of various thick plates of austenitic stainless steel are given in Table 3. CARBON STEEL WELDMENT Carbon steel plates of 17.5 mm thickness were welded with shielded metal arc welding (SMAW) process. Single-V (75 ) butt welding was carried out on these plates as per ASME boiler pressure vessel code (96). After the welding process, the weld plate was machined. Natural defects were incorporated in the weldments by varying the process parameters. The welding process parameters are given in Table 3. DISSIMILAR METAL WELDMENT Austenitic stainless steel type 304 pipe with outside diameter of 60mm and thickness of 5.5mm was welded with carbon steel, ASTM A-106 Grade&B pipe, with outside diameter of 60mm and thickness of 5.5mm with Shielded Metal Arc Welding (SMAW) process single-v (75 ). Butt welding was carried out on this pipe and as per ASME Boiler Pressure Vessel Code (96). After the welding process, the weld area was machined. Natural defects were incorporated in the weldments by varying process parameters. The welding process parameters are given in Table 3. TABLE 3 Welding Process Parameters S. N o Material 1. Austenitic stainless 2. Austenitic stainless 3. Austenitic stainless Thicknes s Joint Geometr y 1 Single 17.5 Single 25.0 Single Process and Polarity Electrod e Spec. 4. Austenitic stainless 36.0 Single Size of electrodes used for welding Current Range (A) editor@iaeme.com

4 Analyzing Defects In Weldments Using Metallography Examination S. N o Joint Geometr y Process and Polarity Electrod e Spec. Size of electrodes used for welding Thicknes Material s 5. Austenitic stainless 6. Austenitic stainless 7. Austenitic stainless 8. Austenitic stainless 40.0 Single 50.0 Single 17.0 Single mm OD and 5.5mm thick Single E L 15 Current Range (A) INCORPORATION OF DEFECTS IN WELDMENTS Natural Defects The 1 mm and 17.5 mm, 36.0 mm and 40.0 mm austenitic stainless steel and the 17.5 mm Carbon steel weld plates had line defects porosities and slag inclusions, etc. These natural defects were indicated by X-ray radiographic testing. These defects were incorporated in weld metal by varying welding parameters like current, welding torch oscillation, and improper cleaning etc. Artificial Defects The 25.0 mm thick austenitic stainless steel weldment was chosen for incorporating artificial defects by, electric discharge machining (EDM) fabricated defects (notches) in it, simulating the natural defects (line defects). Artificial defects such as notches having depth equal to 5,4,3,2,1 percent of plate thickness (parallel to the weld direction) was introduced. These artificial defects are referred to as fine defects in this thesis. These defects are depicted in Fig.1 and Table 4. These notches were made on the top midsection of the weld, along the axis of the weld (longitudinal notches). The details of these defects are shown in Table 4. in 50.0 mm austenitic stainless steel weldment which is free from natural defects, artificial defects were fabricated by drilling three, 3 mm size holes as indicated in Fig.2. The details of these defects are shown in Table editor@iaeme.com

5 Dr.S. Ravichandran ALL DIMENSIONS ARE IN mm Figure 1 Schematic Diagram Showing Locations of Artificial Defects in Austenitic Atainless Steel Weldment of 25mm Thickmess A,B DRILLED HOLES, EACH OF DIAMETER 3mm SIZE. 31mm. DEEP. HOLE C DRILLED DIRECTLY ON OPPOSITE SIDE OF THE WELD CORRESPONDING TO HOLE B OF 3mm SIZE, 31mm DEEP NOT SHOWN. Figure 2 Schematic Diagram Showing Locations of Artificial Defects in Austenitic Atainless Weldment of 50mm Thickmess 18 editor@iaeme.com

6 Analyzing Defects In Weldments Using Metallography Examination Table 4 Size of EDM Fabricated Defects In 25.0 Mm Thick Austenitic Stainless Steel Weldment Type of Defect Depth in mm Width in mm Length in mm 5% Notch 5% of T 0 1 4% Notch 4% of T % Notch 3% of T % Notch 2% of T % Notch 1% of T T Thickness of the weld = 25.0 mm Table 5 Sixe of Artificial Defects In 50.0 Mm Thick Austenitic Stainless Steel Weldments Type of Defect Diameter of holes (mm) Depth (mm) Drilled hole (A) 3 31 Drilled hole (B) 3 31 Drilled hole (C) 3 31 METALLOGRAPHY EXAMINATION The metallography studies on the six austenitic stainless steel plate weldments of thickness 1mm, 17.5mm, 25.0mm, 36.0mm, 40.0mm and 50.0mm sizes were performed to characterize the dendritic microstructure. The dendritic microstructures were observed in all these weldments. The carbon steel plate weldment of 17.5mm thickness was also studied. Pearlite grains with ferrite matrix at grain boundaries were observed. The dissimilar metal weld (Austenitic-ferritic) of pipe was studied to characterize the dendritic microstructure. The dendritic microstructures were observed with little presence of pearlite grains at grain boundaries. The microstructures of austenitic stainless steel weldment of 1mm, 17.5mm thickness are shown in Figs.3(a) and 3(b). RADIOGRAPHY EXAMINATION The radiographic examination of all the six stainless steel weldments and one carbon steel weldment and dissimilar pipe weldments were performed with X-rays. Radiographic examination of these weldments revealed defects like porosities, slag inclusions etc. these results were used to arrive at a basis for examination of the welds of conventional ultrasonic testing and signal processing. IMAGE PROCESSING Image processing of few selected radiographs on certain defects was carried out to appreciate its ability to defect detection and enhanced sensitivity editor@iaeme.com

7 Dr.S. Ravichandran MICROSTRUCTURE OF AUSTENITIC STAINLESS STEEL WELDMENTS Figure 3(a) Microstructure of Austenitic Stainless Steel Weldment of 1mm Thickness PHOTOMICROGRAPH SHOWING WELDMETAL, HAZ AND PARENT METAL Figure 3(b) Microstructure of Austenitic Stainless Steel Weldment of 17.5mm Thickness 20

8 Analyzing Defects In Weldments Using Metallography Examination CONCLUSIONS From the work carried out, the following conclusions are arrived at. The micro structure of austenitic stainless steel welds shows dendritic nature and the extent of dendritic growth in the weld metal is found to be more when the thickness is higher. Such a structure is not found in carbon steel weldment. Dissimilar metal weldments of above materials were also observed with the presence of dendritic growth. REFERENCES [1] C.H.Chen, Signal Processing and Pattern recognition in Non-Destructive Evaluation of Materils, NATO ASI Series F, Vol. 44, Springer-Verlag, (1988), Berlin. [2] P.kalyanasundaram, C. Rajagopalan, Baldev Raj, O.Prabhakar and D.G.R.Sharma, High Sensitivity Detection and Classification of Defects in Austenitic Stainless Steel Weldments using Cluster Analysis and Pattern Recognition, British Journal of NDT, pp , June (1991). [3] ASME Boiler Pressure Vessel Code, Section IX, Qualification standard for Welding and Brazing Procedures, Welders, Brazers and Welding and Brazing operators, (1980). [4] P.kalyanasundaram, C. Rajagopalan, C>V> Subramanian, M.Thavasi Muthu and Baldev Raj, Ultrasonic Signal Analysis for DefectCharacterization in Composite Materials, The British Journal of NDT, pp , May, (1991). AUTHOR PROFILE Dr S. Ravichandran is a Software Quality Professional. He is also a Six Sigma Master Black Belt with academic qualifications ME,MBA,Ph.D.He has carried out software quality appraisals, consultancy and training services in India and over 20 countries such as China, Brazil, Singapore, Malaysia, US, Europe, Middle East etc He is Presently the Chairman, CEO and Chief Scientist of Trimentus Technologies, Chennai & US. He also well known expert in engineering quality including NDT techniques editor@iaeme.com